1. Field of the Invention
The present invention relates to an image recording apparatus utilizing serial recording, and more particularly to an image recording apparatus capable of effecting print control on the end portion of a recording medium such as paper or film, and an image recording method therefor.
2. Related Background Art
FIG. 1 illustrates a conventional image forming apparatus utilizing, for example, the ink jet recording system and adapted for use in a copying machine, a printer or the like.
During the recording operation of said apparatus, the recording medium (hereinafter also called recording sheet) is transported by transport rollers and discharge rollers, each positioned as a vertical pair. The amount of advancement of the recording sheet is equal to the pitch of rows, for example 8 mm. In order to apply a constant tension to the recording sheet, the amount of advancement by the discharge rollers is selected larger, for example by 1%, than that of the transport rollers. Therefore, in the course of recording operation, the recording sheet is advanced by 8 mm if the distance from the transport rollers to the rear end of the sheet is at least equal to 8 mm, as shown in FIG. 2(a).
In the following, the structure of the above-mentioned image forming apparatus will be explained with reference to FIG. 1.
In the bottom portion of a main body 1 of the apparatus, there is accommodated a cassette 3 containing a stack of plural sheets 2. To the left in said drawing, there is provided a carriage 5 supporting a recording head 4, and a platen 6 is provided under said recording head 4.
The recording head 4 is of an ink jet type for recording on the sheet member 2, and includes ink discharge openings 16 provided in a quantity m at the end of an ink discharge portion 15 as shown in FIG. 3. Internally there is provided an unshown ink chamber which serves to discharge ink droplets from the m discharge openings 16 according to image signals. The carriage 5 is linked with an unshown carriage driving motor through a timing belt, and performs reciprocating motion along a guide shaft 5a by said motor.
In order to combine the recording in different rows, the advancement of the sheet member 2 by the lower transport roller 7 has to be conducted with a high precision, in the order of 10 .mu.m. For this purpose the lower transport roller 7 is finished with a precise diameter, and a stepping or pulse motor of a high stopping precision is employed as the driving device and controls the rotational angle of said roller 7 by the number of pulses.
In said apparatus, when a sheet feed roller 11 is rotated in response to a feed signal, an uppermost sheet 2 is separated from the stack and advanced between sheet guides 9, 10.
Being guided by said guides 9, 10, the sheet 2 advances to the nip between the lower transport roller 7, driven by the not shown drive motor, and an upper transport roller 8 which is driven by said lower transport roller 7.
Then the sheet 2 passes on the platen 6 by the transporting force of the lower roller 7 and the upper roller 8 to reach discharge (pulling) rollers 12, 13, and temporarily stops when the leading end is pinched between said rollers 12, 13.
The lower pulling roller 12 rotates in linkage with the lower transport roller 7 but has a somewhat larger peripheral speed, and the pinching force of the rollers 12, 13 on the sheet 2 is selected weaker than that of the transport rollers 7, 8, whereby the sheet 2 is maintained without slack under a suitable tension.
In this state the recording head 4 supported by the carriage 5 moves in a direction away from the viewer in FIG. 1 and discharges ink according to the image signals, thereby forming a recording of a predetermined width (recording width) on the sheet member 2. Said recording width W is represented by m.times.d, wherein m is the number of ink discharge openings and d is the diameter of a dot.
After recording of each row, the sheet 2 is advanced by the recording width by means of the transport rollers 7, 8, and then the recording of a next row is conducted. The details of the transport mechanism are shown in FIG. 6.
The recording on the sheet 2 is conducted by the repetition of the above-explained operations, and, upon completion of recording of a sheet, the sheet 2 is discharged from the discharge rollers 12, 13 onto a discharge tray 14. FIG. 4 illustrates an example of the image recorded on the sheet 2.
FIG. 5 shows another example of transport mechanism of the conventional recording apparatus, in which transport rollers 7, 8 are positioned at the downstream side of the transport path for the sheet 2, with respect to the recording head 4, while pulling rollers 17, 18 are positioned at the upstream side. The pulling rollers 17, 18 have a transport amount somewhat smaller than that of the lower transport roller 7, and have a pinching force on the sheet 2 weaker than that of the transport rollers 7, 8, whereby the sheet 2 is maintained without slack under a suitable tension.
In the above-explained apparatus, however, in case the distance from the transport rollers to the rear end of sheet is less than 8 mm as shown in FIG. 2(b), the amount of sheet advancement becomes larger than 8 mm, (8 mm+.alpha.), because the rear end of the sheet is released from the transport rollers in the course of sheet advancement as shown in FIG. 2(c) and the sheet is advanced thereafter by the discharge rollers only. Such fluctuation in the amount of sheet advancement may result in a deviation in the position of records.
More specifically, in the transport mechanism shown in FIG. 6, the transport rollers 7, 8 are positioned at the upstream side of the transport path with respect to the recording head 4, while the pull rollers 12, 13 are positioned at the downstream side, and the amount of advancement by said pull rollers 12, 13 is selected somewhat larger than that by the transport rollers 7, 8. After the rear end of the sheet 2 is released by the transport rollers 7, 8, the sheet 2 is transported by the pull rollers 12, 13 only. Consequently, with a motor rotating angle for the normal sheet advancement, the sheet is advanced more than it is released from the transport rollers, so that a high precise amount of advancement, in the order of 10 .mu.m cannot be maintained.
In case a same stepping motor is used for driving the transport rollers and the pull rollers, and if said stepping motor rotates by a same amount without detecting the release of the sheet from the transport rollers, the sheet is advanced by a larger amount corresponding to the larger speed of the pull rollers.
For example, if the advancement of 8 mm is conducted by 100 pulses and if the pull rollers have a peripheral speed larger by 1%, said pull rollers advance the sheet by 8.times.1.01=8.08 mm corresponding to 100 pulses, thus resulting in an excessive advancement by 80 microns.
Consequently the high precision recording on the sheet 2 is possible only while the sheet 2 can be precisely advanced by the lower transport roller 7. Therefore, the highly precise recording has to be completed until the sheet 2 is released by the transport rollers 7, 8 so that, as shown in FIG. 6, the blank area x at the rear end of the sheet 2 becomes inevitably large.
Also in the transport mechanism shown in FIG. 5, in which the transport rollers 7, 8 are positioned at the downstream side of the transport path with respect to the recording head 4 while the pull rollers 17, 18 are positioned at the upstream side, wherein the amount of transportation by said pull rollers 17, 18 is selected to be somewhat less than that of the transport rollers 7, 8, the amount of advancement of the sheet becomes less for a same amount of motor rotation, until the leading end of the sheet 2 is pinched by the transport rollers 7, 8. Consequently the highly precise recording has to be completed while the sheet 2 is transported by the pull rollers 17, 18 only, so that, as shown in FIG. 5, the blank margin x' at the leading end of the sheet 2 becomes inevitably large.
In the following there will be further considered the range enabling high precision recording, with reference to FIG. 7.
For a sheet advancement of 20 mm at the start of recording and a sheet advancement of 8 mm at each step, the amount transportable by the transport rollers at the last step is correlated with the length of the sheet, as shown in FIG. 7B. Therefore, in order to vary the amount of advancement in the last step, there is required means for detecting the transportable amount.
For this purpose use can be made of detection means shown in FIG. 8, consisting of a sensor arm 19-1 and a transmission sensor 19-2 for detecting the rear end of the sheet in the course of transportation thereof. Since the motor rotation passes through stages of acceleration, constant speed and deceleration as shown in FIG. 9 in the one-step advancement of the sheet 2, there is required a certain time for the sensor arm 19-1 to rotate to a position 19. Thus, if the rear end of the sheet 2 leaves the sensor arm 19-1 at a position A in FIG. 9, the sensor arm 19-1 rotates to the position 19 only at a time B. Thus generated is an error indicated by the hatched area, and the remaining amount at the rear end, calculated from the detection of the rear end of sheet, becomes a significantly different from the actual amount. This relation is shown in FIG. 10. As the detection means involves a significant error as explained above, the sheet advancement cannot be conducted with the transportable amount for the last step shown in FIG. 7B so that a large blank area at the rear end of the sheet has been unavoidable.